1. Field of the Invention
This invention relates generally to radio systems for monitoring the whereabouts of objects including individuals, animals and articles and more particularly is directed to a system which includes a transponder attached to the object being monitored and a monitoring base station which responds to a loss of communication with the transponder, the system being capable of finding the transponder locally by homing and utilizing networks of commercial radio stations and public safety radio systems to localize the position of a missing transponder on a nationwide basis to within an area sized for finding by local homing techniques.
2. Description of the Prior Art
The problem of monitoring the removal of persons and articles from prescribed areas has been addressed and solved by various systems and radio devices.
U.S. Pat. No. 4,023,138 to Ballin discloses a vehicle theft prevention system in which a receiver on the vehicle receives a signal from a remote transmitter and detects a drop in signal strength caused by an increase in the distance between the vehicle and the transmitter and actuates an alarm on the vehicle or disables the latter.
U.S. Pat. No. 4,593,273 to Narcisse discloses an out-of-range monitor and alarm system for supervised persons which includes a base unit transmitting a signal to a mobile unit carried by the person being supervised. The mobile unit responds to a threshold circuit which is adjustable to a prescribed distance between the base and supervised person. When the distance is exceeded as indicated by a drop in signal strength, a transmitter in the mobile unit signals the base to indicate the straying of the supervised person beyond acceptable limits. The mobile unit may also sound an alarm to alert the supervised person that he is straying beyond the prescribed area.
U.S. Pat. No. 4,598,272 to Cox discloses a child monitoring device comprising two portable units, each having a radio transceiver and antenna. One of the units is secured to the child's person the other is carried by the parent. The child's unit transmits a reference signal and can receive a different, locator signal and has an audible alarm speaker which can be actuated by the locator signal. The parent's unit receives the reference signal from the child's transmitter and, by threshold detection, sounds a beep when the child's location exceeds a predetermined distance. A light signal in the parent's unit remains lit as an indication that the child's unit is transmitting and that the threshold conditions have not been met. When the beep in the parent's unit indicates wandering of the child, the parent, by pressing a button, can actuate transmission of the locator signal, which is stronger than the reference signal, to sound a raucous alarm in the child's unit to indicate the whereabouts of the child. Clearly, this device is applicable only to very special situations and is intended to operate in a limited locality.